Faster fault-detection mechanism, for example using bidirectional forwarding detection (BFD), on network nodes and/or hosts multihomed using a link aggregation group (LAG)

ABSTRACT

For use in a system including a first data forwarding device, a second data forwarding device, a third data forwarding device, a first communications link between the first data forwarding device and the second data forwarding device, and a second communications link between the first data forwarding device and the third data forwarding device, the first and second communications links belonging to a link aggregation group (LAG), a method includes (1) generating a message (i) for testing a first path between the first data forwarding device and the second data forwarding device, and a second path between the first data forwarding device and the third data forwarding device, and (ii) including an Internet protocol (IP) datagram including a multicast IP destination address and a payload containing path testing information; and (2) sending, over the LAG, the message from the first data forwarding device to both the second data forwarding device and the third data forwarding device. Responsive to receiving an instance of the message by either of the second or third data forwarding device, such device(s) (1) determine whether or not the received instance of the message is a fault detection on a multihomed link aggregation group message, and (2) processing the received instance of the message based on the determination of whether or not it is a fault detection on a multihomed link aggregation group message.

§ 0. RELATED APPLICATION(S)

The present application is a continuation of pending U.S. patentapplication Ser. No. 16/399,841 (referred to as “the '841 application”and incorporated herein by reference), filed on Apr. 30, 2019, titled“FASTER FAULT-DETECTION MECHANISM, FOR EXAMPLE USING BIDIRECTIONALFORWARDING DETECTION (BFD), ON NETWORK NODES AND/OR HOSTS MULTIHOMEDUSING A LINK AGGREGATION GROUP (LAG)” and listing Surya Prakash S.Kalburgi as the inventor.

§ 1. BACKGROUND OF THE INVENTION § 1.1 Field of the Invention

The present description concerns communications networks. Morespecifically, the present description concerns detecting faults incommunications networks, especially in communications networks havingmultihomed access, such as via Ethernet Virtual Private Network (EVPN)multihoming for example.

§ 1.2 Background Information

§ 1.2.1 Multihoming Examples and Technologies Supporting Multihoming

In an example datacenter environment within the cloud, one designconsideration for network architects is that the networks be moreresilient and fault tolerant. The desire for resiliency and faulttolerance spans across all or most segments of the cloud-baseddatacenter; from the host where applications are hosted, to a wide areanetwork (WAN) peering with Internet.

In typical datacenter design, the end host is usually connected tomultiple Top of Rack (TOR) nodes (also referred to as “leaf nodes”)using Link-Aggregation-Group (LAG) bundles. This multihomed architectureprovides fault-tolerant connectivity between the leaf nodes and the hostin the event of a single-link failure as compared to single-homed host(in which the host is connected to just a single leaf node). Since it isdesired to switch over any traffic from the failed link (between thehost and one of the leaf nodes) to an available link (between the hostand another of the left nodes) quickly, it is important to detect thefault (and to switch over) as quickly as possible.

Another example in which multihoming is useful is when a customer edge(CE) device is connected to two or more provider edge (PE) devices toprovide redundant connectivity.

Some of the common implementations supporting multihomed networkarchitectures include Ethernet Virtual Private Network (EVPN) EthernetSegment Identifier (ESI) based multihoming and Multi-Chassis-LAG(MC-LAG). Although these technologies are understood, they are brieflyintroduced below for the reader's convenience.

Different modes of operation for multihoming include “single,”“active-standby” and “active-active.” When a device is connected to asingle-homed device (or site), the mode of operation is “single.” Whenonly a single device among a group of devices (e.g., attached to anEthernet segment) is allowed to forward traffic to and from anotherdevice (e.g., on that Ethernet segment), the mode of operation is the“active-standby” redundancy mode. Finally, when all (or at leastmultiple) devices of a group (e.g., attached to an Ethernet segment) areallowed to forward traffic to and from another device (e.g., on thatEthernet segment), the mode of operation is the “active-active” (e.g.,load balanced) redundancy mode.

As just noted above, multihoming is useful is when a CE device isconnected to two or more PE devices to provide redundant connectivity.An Ethernet Virtual Private Network (EVPN) may be used to support such atopology. For example, EVPN may include CE devices that are connected toPE devices, which form the edge of the multiprotocol label switching(MPLS) infrastructure (sometimes referred to as an MPLS transportnetwork). A CE device can be a host, a router, or a switch. The PEdevices provide Layer 2 virtual bridge connectivity between the CEdevices. There can be multiple EVPNs in the provider's transportnetwork. Learning between the PE routers occurs in the control planeusing, for example, the border gateway protocol (BGP), unliketraditional bridging, where learning occurs in the data plane. The EVPNmultihoming feature enables a network administrator to connect acustomer site to two or more PE devices to provide redundantconnectivity. A CE device can be multihomed to different PE devices orthe same PE device. A redundant PE device can provide network service tothe customer site as soon as a failure is detected. Thus, EVPNmultihoming helps to maintain EVPN service and traffic forwarding to andfrom the multihomed site in the event of (a) PE device to CE device linkfailure, (b) PE device failure, and/or (c) MPLS-reachability failurebetween the local PE device and a remote PE device.

IEEE 802.3ad link aggregation enables network administrators to groupEthernet interfaces to form a single link layer interface, also known asa link aggregation group (LAG) or bundle.

Aggregating multiple links between physical interfaces creates a single,logical, point-to-point trunk link or a LAG. The aggregated Ethernetinterfaces that participate in a LAG are called “member links.” LAGs canbe used to increase bandwidth, provide graceful degradation as failureoccurs, and/or increase availability. LAGs can provide networkredundancy by load-balancing traffic across all available links. If oneof the links fails, the system automatically load-balances trafficacross all remaining links. That is, since a LAG is composed of multiplemember links, even if one member link fails, the LAG can continue tocarry traffic over the remaining links.

The Link Aggregation Control Protocol (LACP), a component of IEEE802.3ad, provides additional functionality for LAGs. More specifically,LACP can be used to ensure that both ends of the Ethernet link arefunctional and are members of the aggregation group before the link isadded to the LAG. When LACP is configured, it detects misconfigurationson the local end or the remote end of the link. Thus, LACP can help toprevent communication failure.

One problem with standard link aggregation is that the connections arepoint to point. Multichassis link aggregation groups (MC-LAGs) enable aclient device to form a logical LAG interface between two MC-LAG peers.An MC-LAG provides redundancy and load balancing between the two MC-LAGpeers, multihoming support, and a loop-free layer 2 network withoutneeding to run the spanning tree protocol (STP). On one end of anMC-LAG, there is an MC-LAG client device, such as a server, that has oneor more physical links in a link aggregation group (LAG). This clientdevice uses the link as a LAG. On the other side of the MC-LAG, therecan be (e.g., a maximum of) two MC-LAG peers. Each of the MC-LAG peershas one or more physical links connected to a single client device.

The MC-LAG peers may use the Inter-Chassis Control Protocol (ICCP) toexchange control information and coordinate with each other to ensurethat data traffic is forwarded properly. LACP may be used to discovermultiple links from a client device connected to an MC-LAG peer.

§ 1.2.2 Limitations of Using the Link Aggregation Control Protocol(LACP) for Fault Detection

In the current deployments, the Link Aggregation Control Protocol (LACP)is used as an operations, administration and management (OAM) protocolto detect and isolate any fault on the connected interfaces.Unfortunately, however, even with fastest timeout interval set on LACP,it takes about 3-5 seconds to detect the fault along the interface. Sucha 3-5 second delay can lead to service degradation, which can impact endapplications.

FIGS. 1A-1C illustrate an example datacenter environment 100 used toillustrate the foregoing problem. The example environment includes adatacenter having leaf nodes (L1-Ln) 120 which are connected withspine/core nodes (N1-Nm) 130 directly, or via a distribution layer 140of nodes. Assume an EVPN ESI deployment in which (1) host H1 isconnected to leaf node L1, (2) the leaf node L1 has EVPN peering withtwo other leaf nodes L2 and L3, and (3) host H2 is multihomed to L2 andL3 using LACP.

Referring to FIG. 1A, in a steady state with all links up, traffic fromhost H1 to host H2 gets load-balanced between leaf nodes L2 and L3 asindicated by the arrowed paths. Referring next to FIG. 1B, assume thatthe link between leaf node L3 and host H2 becomes faulty due tocongestion or a state which impairs node from processing the packets.(Note that this can occur even if the physical link is UP. For example,in some cases, the physical link can be operational, but the OAMprotocol can go down due to congestion, in which case the remote deviceis in a state in which OAM packets are not processed.) As a result,traffic from host H1 to host H2 that gets load-balanced on leaf node L3would “blackhole” for at least 3-5 seconds until LACP detects theunderlying problem and isolates the link from LAG bundles. This is alsotrue for the traffic in reverse direction from host H2 to host H1, thatgets load-balanced on H2 and sprayed across the links connected to leafnode L2 and leaf node L3. As shown in FIG. 1C, once LACP times out andisolates the faulty link, all of the traffic between host H1 and host H2is now forwarded via leaf node L2.

In the foregoing scenario (and similar situations in which a device ismultihomed to different nodes and LACP is used), a faster sub-secondfault detection method is highly desired to enable quicker isolation ofa faulty interface. Note that a “faulty interface” is intended to beinterpreted broadly and include, for example, a fault in the line cardassociated with the interface, a fault in the physical link terminatedby the interface, etc. In this context the faulty interface relates to astate when the traffic on the interface in either of the direction getscorrupted or dropped due to multiple issues but not limited to commonscenarios like data transmission, impaired state on leaf nodes or hostnodes, or congestion. Thus, if the faulty interface is detected andisolated within sub-seconds, all (or most) traffic from host H1 to hostH2 and vice-versa would be diverted through leaf node L2 without any (orwith minimal) visible impact on the services and applications.

§ 1.2.3 Using Bidirectional Forwarding Detection (BFD) for Faster FaultDetection

The document “Bidirectional Forwarding Detection (BFD),” Request forComments 5880 (Internet Engineering Task Force, June 2010) (referred toas “RFC 5880” and incorporated herein by reference) notes that the useof protocol “Hellos” to detect a failure in the path between two routingprotocol engines is too slow in certain situations. RFC 5880 describes amethod for providing low-overhead, short-duration detection of failuresin the path between adjacent forwarding engines (including theinterfaces, data link(s), and, to the extent possible, the forwardingengines themselves).

Per RFC 5880, BFD is designed to detect failures in communication with aforwarding plane next hop. It is intended to be implemented in somecomponent of the forwarding engine of a system, in cases where theforwarding and control engines are separated. BFD may also beimplemented in the control engine, though doing so may preclude thedetection of some kinds of failures. BFD operates on top of any dataprotocol (network layer, link layer, tunnels, etc.) being forwardedbetween two systems. It is always run in a unicast, point-to-point mode.(Consequently, the current implementation of BFD is not useful, or atleast not specified, for multihomed connections.) BFD packets arecarried as the payload of whatever encapsulating protocol is appropriatefor the medium and network. BFD may be running at multiple layers in asystem and the context of the operation of any particular BFD session isbound to its encapsulation. BFD can provide failure detection on anykind of path between systems, including direct physical links, virtualcircuits, tunnels, MPLS Label Switched Paths (LSPs), multihop routedpaths, and unidirectional links (provided some return path exists).Multiple BFD sessions can be established between the same pair ofsystems when multiple paths between them are present in at least onedirection, even if a lesser number of paths are available in the otherdirection.

Per RFC 5880, the BFD state machine implements a three-way handshake,both when establishing a BFD session and when tearing it down for anyreason, to ensure that both systems are aware of the state change. BFDcan be abstracted as a simple service. The service primitives providedby BFD are to create, destroy, and modify a session, given thedestination address and other parameters. BFD in return provides asignal to its clients indicating when the BFD session goes up or down.

More specifically, per RFC 5880, BFD is a simple Hello protocol that, inmany respects, is similar to the detection components of well-knownrouting protocols. A pair of systems transmit BFD Echo and/or BFDControl packets (referred to more generally as “BFD packets”)periodically over each path between the two systems. If a system stopsreceiving BFD packets for a long enough time period, some component inthat particular bidirectional path to the neighboring system is assumedto have failed. Under some conditions, systems may negotiate not to sendperiodic BFD packets in order to reduce overhead. A path is onlydeclared to be operational when two-way communication has beenestablished between systems, though this does not preclude the use ofunidirectional links. As noted above, a separate BFD session may becreated for each communications path and data protocol in use betweentwo systems.

Per RFC 5880, each system estimates how quickly it can send and receiveBFD packets in order to come to an agreement with its neighbor (alsoreferred to as “peer”) about how rapidly detection of failure will takeplace. These estimates can be modified in real time in order to adapt tounusual situations. This design also allows for fast systems on a sharedmedium with a slow system to be able to more rapidly detect failuresbetween the fast systems while allowing the slow system to participateto the best of its ability.

The document, “Bidirectional Forwarding Detection (BFD) for IPv4 andIPv6 (Single Hop),” Request for Comments 5881 (Internet Engineering TaskForce, June 2010) (referred to as “RFC 5881” and incorporated herein byreference) describes using BFD to track IPv4 and IPv6 connectivitybetween directly connected systems. It describes the encapsulation ofBFD control packets within User Datagram Protocol (UDP) packets, whichare transmitted in IPv4 or IPv6 packets.

Presently (early 2019), BFD is a very popular OAM protocol due to itsease of operation. As noted, BFD relies on plain IP/UDP, which worksacross all protocols.

§ 1.2.4 Using BFD for Fault Detection on a Per Link Basis in a LinkAggregation Group (LAG) Bundle

The document “Bidirectional Forwarding Detection (BFD) on LinkAggregation Group (LAG) Interfaces,” Request for Comments 7130 (InternetEngineering Task Force, February 2014) (referred to as “RFC 7130” andincorporated herein by reference) describes enabling BFD on a per-linkbasis in an LAG bundle. RFC7130 specifies that a destination IP addressmust be configured on the peer and should be reachable through the LAGinterface, but does not provide any clarity on the specifics ofdestination IP address, thereby permitting different implementations.Consequently, BFD on LAG as per RFC 7130 would not be suitable for a LAGdeployment in a multihomed environment, such as the one discussed abovewith reference to FIGS. 1A-1C (since the two (leaf) nodes would have twodifferent destination IP addresses).

These limitations and/or requirements of RFC 7130 shortcomings cause anumber of problems. First, different equipment vendors have differentimplementations regarding the destination IP address. This raises theneed for specific configuration(s) to enable interoperability betweendifferent equipment vendors. Second, any change in the IP address wouldrequire relevant configuration changes across all participating nodes.Third, in a multihomed LAG (such as, for example, the one illustrated inFIGS. 1A-1C), the destination IP address can't be same for all the linksbecause each link would terminate on different (e.g., leaf) nodes, andsuch different nodes don't necessarily have same IP address configured.A similar problem occurs for a CE device multihomed (e.g., with a LAG)to PE devices at the edge of a provider transport network.

Thus, it would be useful to detect faults more quickly (e.g., morequickly than 3-5 seconds as in LACP, and preferably with sub-secondtiming), especially in multihomed environments, and especially inMC-LAGs.

§ 2. SUMMARY OF THE INVENTION

For use in a system including a first data forwarding device, a seconddata forwarding device, a third data forwarding device, a firstcommunications link between the first data forwarding device and thesecond data forwarding device, and a second communications link betweenthe first data forwarding device and the third data forwarding device,the first and second communications links belonging to a linkaggregation group (LAG), an example method consistent with the presentdescription includes (1) generating a message (i) for testing a firstpath between the first data forwarding device and the second dataforwarding device, and a second path between the first data forwardingdevice and the third data forwarding device, and (ii) including anInternet protocol (IP) datagram including a multicast IP destinationaddress and a payload containing path testing information; and (2)sending, over the LAG, the message from the first data forwarding deviceto both the second data forwarding device and the third data forwardingdevice.

Responsive to receiving an instance of the message by either of thesecond or third data forwarding device, such device(s) (1) determinewhether or not the received instance of the message is a fault detectionon a multihomed link aggregation group message, and (2) processing thereceived instance of the message based on the determination of whetheror not it is a fault detection on a multihomed link aggregation groupmessage.

In some such example methods, the message is carried in an Ethernetframe. The Ethernet frame may include a multicast destination mediaaccess control (MAC) address. For example, the MAC address may be01-00-5E-90-00-01.

In some such example methods, the path testing information in thepayload of the message includes at least one of (A) a bidirectionalforwarding detection (BFD) control packet compliant with RFC 5880 or (B)a BFD echo packet compliant with RFC 5880. In some such example methods,the path testing information in the payload of the message includes atleast one of (A) a bidirectional forwarding detection (BFD) controlpacket compliant with RFC 7130 or (B) a BFD echo packet compliant withRFC 7130.

In some such example methods, the act of testing the message forcompliance with the path testing procedure includes checking thedestination IP address of the message.

In some such example methods, the path testing information in thepayload of the message includes at least one of (A) a bidirectionalforwarding detection (BFD) control packet compliant with RFC 5880 or (B)a BFD echo packet compliant with RFC 5880, and the path testingprocedure is bidirectional forwarding detection in accordance with RFC5880.

In some such example methods, the path testing information in thepayload of the message includes at least one of (A) a bidirectionalforwarding detection (BFD) control packet compliant with RFC 5880 or (B)a BFD echo packet compliant with RFC 5880, and the path testingprocedure is bidirectional forwarding detection in accordance with RFC7130.

Any of the foregoing example methods may be implemented on a dataforwarding device, such as a router for example.

Any of the foregoing example methods may be stored asprocessor-executable instructions on a non-transitory computer-readablestorage medium.

§ 3. BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C illustrate an example datacenter environment used toillustrate the challenge of quickly detecting link failures in amultihomed environment, such as in a MC-LAG.

FIG. 2 illustrates a datacenter environment in which a techniquecompliant with RFC 7130 is used to check connectivity of a host devicemultihomed to leaf nodes of a datacenter.

FIG. 3 is an example method for providing faster fault-detection (e.g.,using bidirectional forwarding detection (BFD)) on multihomed networknodes and/or hosts using a LAG in a manner consistent with the presentdescription.

FIG. 4 is an example Ethernet frame which may be used as a BFD onmulti-homed LAG message consistent with the present description.

FIG. 5 is an example Ethernet payload including an IP datagram, a UDPdatagram and a BFD control packet or BFD echo packet.

FIG. 6 illustrates an example environment including two systems coupledvia communications links.

FIG. 7 is a block diagram of an example router on which the examplemethods of the present description may be implemented.

FIG. 8 is a block diagram of example distributed application specificintegrated circuits (“ASICs”) that may be provided in the example routerof FIG. 8.

FIGS. 9A and 9B illustrate example packet forwarding operations of theexample distributed ASICs of FIG. 8.

FIG. 10 is a flow diagram of an example packet forwarding method thatmay be implemented on any of the example routers of FIGS. 9 and 10.

FIG. 11 is a block diagram of an example processor-based system that maybe used to execute the example method(s) consistent with the presentdescription.

§ 4. DETAILED DESCRIPTION

The present description may involve novel methods, apparatus, messageformats, and/or data structures for detecting faults more quickly (e.g.,more quickly than 3-5 seconds as in LACP, and preferably with sub-secondtiming), especially in multihomed environments, and especially inMC-LAGs. The following description is presented to enable one skilled inthe art to make and use the invention, and is provided in the context ofparticular applications and their requirements. Thus, the followingdescription of embodiments consistent with the present inventionprovides illustration and description, but is not intended to beexhaustive or to limit the present invention to the precise formdisclosed. Various modifications to the disclosed embodiments will beapparent to those skilled in the art, and the general principles setforth below may be applied to other embodiments and applications. Forexample, although a series of acts may be described with reference to aflow diagram, the order of acts may differ in other implementations whenthe performance of one act is not dependent on the completion of anotheract. Further, non-dependent acts may be performed in parallel. Noelement, act or instruction used in the description should be construedas critical or essential to the present invention unless explicitlydescribed as such. Also, as used herein, the article “a” is intended toinclude one or more items. Where only one item is intended, the term“one” or similar language is used. Thus, the present invention is notintended to be limited to the embodiments shown and the inventor regardshis invention as any patentable subject matter described.

§ 4.1 Example Method(s)

FIG. 2 is an example environment 200, similar to that 100 of FIGS.1A-1C, in which example methods consistent with the present descriptioncan be used. Note that host device H2 is multihomed to leaf nodes L2 andL3, and that the example methods may be used in accordance with (e.g.,in compliance with) BFD on LAG per RFC 7130. Further, the links maybelong to a LAG.

FIG. 3 is an example method 300 for providing faster fault-detection(e.g., using bidirectional forwarding detection (BFD)) on network nodesand/or hosts multihomed using a LAG in a manner consistent with thepresent description. Different branches of the example method 300 areperformed in response to the occurrence of different events. (Eventbranch point 310) Referring first to the left branch of the examplemethod 300, in response to it being time to generate a fault detectionprotocol (e.g., BFD compliant) message (e.g., a BFD packet), the examplemethod 300 may generate a fault detection (e.g., BFD) on multihomed LAGmessage (e.g., packet). (Block 320) If the fault detection protocol isto be BFD, the message should be compliant with RFC 7130. Exampleformats for this message are described in § 4.2 below, with reference toFIGS. 4 and 5. As described in more detail later, the message shouldhave both (1) a multicast media access control (MAC) address and (2) amulticast destination IP address. The message is then sent to peerdevices (Block 330) before the example method returns (Node 390).

Referring back to event branch point 310, referring to the right branchof the example method 300, in response to receiving a fault detectionprotocol (e.g., BFD-compliant) message (packet), the example method 300determines whether the message includes both (1) a multicast MAC addressand (2) a multicast IP address. (Decision 350) If so (Decision 350,YES), the example method 300 identifies the message as a fault detection(e.g., BFD) on a multihomed LAG message (Block 360) and processes themessage accordingly (Block 370) before the method returns (Node 390).If, on the other hand, the message does not include both (1) a multicastMAC address and (2) a multicast IP address (Decision 350, NO), theexample method 300 processes the message otherwise (e.g., in aconventional manner) (Block 380) before the example method 300 returns(Node 390). Note that in any protocol definition, there would be clearguidelines on how to identify a particular control packet. The MAC andIP destination addresses are checked to classify the message (e.g.,packets) to avoid any ambiguity in differentiating between “regular” and“multi-homed” fault detection.

As can be appreciated from the foregoing, in the context of BFD, when aBFD packet arrives, the destination MAC address (e.g., of01-00-5E-90-00-01) indicates it is BFD on LAG (e.g., compliant with RFC7130) and the multicast destination IP address further classifies itinto a multihomed LAG bundle. UDP and BFD parameters could remainunchanged as per the RFC 7130 definition.

Referring back to block 320, in the context of BFD, the BFD onmultihomed LAG message may be formatted, for example, as described inwith reference to FIGS. 4 and 5 in § 4.2 below.

§ 4.2 Example Data Structure(S)

FIG. 4 is an example Ethernet frame 400 which may be used as the BFD onmulti-homed LAG message. As shown, the example Ethernet frame 400 mayinclude a 58-bit preamble 410, a 6-bit start of frame delimiter field420, a 48-bit destination MAC address field 430, a 48-bit source MACaddress field 440, a 16-bit type field 450, payload 460, and a 32-bitframe check sequence (FCS) field 470.

Referring back to field 430, the 48-bit destination MAC address may beset to 01-00-5E-90-00-01. This destination MAC address is specified in §2.3 of RFC 7130.

The payload 460 may include an IP datagram including a UDP datagram,with the UDP datagram including a BFD control packet or BFD echo packet.An example payload 460′ is now described with reference to FIG. 5. Theexample payload 460′ basically includes an IP datagram 510, a UDPdatagram 550 and a BFD control packet or BFD echo packet (e.g., per RFC5880) 590.

As shown, the IP datagram 510 may include a 4-bit version field 512, afour bit internet header length (IHL) field 514, an 8-bit type ofservice field 516, a 16-bit total length field 518, a 16-bitidentification field 520, a 3-bit flags field 522, a 13-bit fragmentoffset field 524, an 8-bit time to live (TTL) field 526, an 8-bitprotocol field 528, a 16-bit header checksum field 530, a 32-bit sourceaddress field 532, a 32-bit destination address field 534, a 24-bitoptions field 536 and an 8-bit padding field 538.

Referring back to field 534, the destination address field may carryaddress value 224.0.0.120. It may be some other multicast addressinstead, though preferably one available (not otherwise reserved) perthe Internet Assigned Numbers Authority (IANA). Using a multicastdestination IP address supports BFD on a multihomed environment byensuring that the destination IP address used to send the BFD packetwill be accepted even without being configured on the peer node.

Referring back to field 532, it may be advisable to have the source IPaddress be the logical IP address attached to chassis or host.

The UDP datagram 550 may include a 16-bit source port field 552, a16-bit destination port field 554, a 16-bit length field 556 and a16-bit checksum field 558. Referring back to field 554, the destinationport may be set to 6784 per § 2.2 of RFC 7130.

Referring back to decision 350 of the example method 300 of FIG. 3, themulticast destination IP address (e.g., 224.0.0.120) permits the node tounderstand that the message is for a multihomed LAG bundle, and themulticast MAC address (e.g., 01-00-5E-90-00-01) is compliant with RFC7130. These two multicast address permit a node to accept and identify aprotocol (e.g., BFD) message on multihomed LAG. This identification andclassification can be used to exchange link state information betweenlayer 2 (L2) and layer 3 (L3) protocols.

§ 4.3 Example Apparatus

FIG. 6 illustrates two data forwarding systems 610 and 620 coupled viacommunications links 630. The links may be physical links or “wireless”links. The data forwarding systems 610,620 may be nodes, such as routersfor example. If the data forwarding systems 610,620 are example routers,each may include a control component (e.g., a routing engine) 614,624and a forwarding component 612,622. Each data forwarding system 610,620includes one or more interfaces 616,626 that terminate one or morecommunications links 630. Any two of the label switched routers (LSRs)may be implemented on device 610 and/or 620. The example method 300described above may be implemented in the control component 614 and/or624 of device 610 and/or 620.

As just discussed above, and referring to FIG. 7, some example routers700 include a control component (e.g., routing engine) 710 and a packetforwarding component (e.g., a packet forwarding engine) 790.

The control component 710 may include an operating system (OS) kernel720, routing protocol process(es) 730, label-based forwarding protocolprocess(es) 740, interface process(es) 750, user interface (e.g.,command line interface) process(es) 760, and chassis process(es) 770,and may store routing table(s) 739, label forwarding information 745,and forwarding (e.g., route-based and/or label-based) table(s) 780. Asshown, the routing protocol process(es) 730 may support routingprotocols such as the routing information protocol (“RIP”) 731, theintermediate system-to-intermediate system protocol (“IS-IS”) 732, theopen shortest path first protocol (“OSPF”) 733, the enhanced interiorgateway routing protocol (“EIGRP”) 734 and the border gateway protocol(“BGP”) 735, and the label-based forwarding protocol process(es) 740 maysupport protocols such as BGP 735, the label distribution protocol(“LDP”) 736 and the resource reservation protocol (“RSVP”) 737. Therouting protocol process(es) 730 (or associated processes for teachingresulting routes) may implement the example method 300 of FIG. 3. One ormore components (not shown) may permit a user 765 to interact with theuser interface process(es) 760. Similarly, one or more components (notshown) may permit an outside device to interact with one or more of therouter protocol process(es) 730, the label-based forwarding protocolprocess(es) 740, the interface process(es) 750, and the chassisprocess(es) 770, via SNMP 785, and such processes may send informationto an outside device via SNMP 785.

The packet forwarding component 790 may include a microkernel 792,interface process(es) 793, distributed ASICs 794, chassis process(es)795 and forwarding (e.g., route-based and/or label-based) table(s) 796.

In the example router 700 of FIG. 7, the control component 710 handlestasks such as performing routing protocols, performing label-basedforwarding protocols, control packet processing, etc., which frees thepacket forwarding component 790 to forward received packets quickly.That is, received control packets (e.g., routing protocol packets and/orlabel-based forwarding protocol packets) are not fully processed on thepacket forwarding component 790 itself, but are passed to the controlcomponent 710, thereby reducing the amount of work that the packetforwarding component 790 has to do and freeing it to process packets tobe forwarded efficiently. Thus, the control component 710 is primarilyresponsible for running routing protocols and/or label-based forwardingprotocols, maintaining the routing tables and/or label forwardinginformation, sending forwarding table updates to the packet forwardingcomponent 790, and performing system management. The example controlcomponent 710 may handle routing protocol packets, provide a managementinterface, provide configuration management, perform accounting, andprovide alarms. The processes 730, 740, 750, 760 and 770 may be modular,and may interact with the OS kernel 720. That is, nearly all of theprocesses communicate directly with the OS kernel 720. Using modularsoftware that cleanly separates processes from each other isolatesproblems of a given process so that such problems do not impact otherprocesses that may be running. Additionally, using modular softwarefacilitates easier scaling.

Still referring to FIG. 7, the example OS kernel 720 may incorporate anapplication programming interface (“API”) system for external programcalls and scripting capabilities. The control component 710 may be basedon an Intel PCI platform running the OS from flash memory, with analternate copy stored on the router's hard disk. The OS kernel 720 islayered on the Intel PCI platform and establishes communication betweenthe Intel PCI platform and processes of the control component 710. TheOS kernel 720 also ensures that the forwarding tables 796 in use by thepacket forwarding component 790 are in sync with those 780 in thecontrol component 710. Thus, in addition to providing the underlyinginfrastructure to control component 710 software processes, the OSkernel 720 also provides a link between the control component 710 andthe packet forwarding component 790.

Referring to the routing protocol process(es) 730 of FIG. 7, thisprocess(es) 730 provides routing and routing control functions withinthe platform. In this example, the RIP 731, ISIS 732, OSPF 733 and EIGRP734 (and BGP 735) protocols are provided. Naturally, other routingprotocols may be provided in addition, or alternatively. Similarly, thelabel-based forwarding protocol process(es) 740 provides labelforwarding and label control functions. In this example, the LDP 736 andRSVP 737 (and BGP 735) protocols are provided. Naturally, otherlabel-based forwarding protocols (e.g., MPLS) may be provided inaddition, or alternatively. In the example router 700, the routingtable(s) 739 is produced by the routing protocol process(es) 730, whilethe label forwarding information 745 is produced by the label-basedforwarding protocol process(es) 740.

Still referring to FIG. 7, the interface process(es) 750 performsconfiguration of the physical interfaces (Recall, e.g., 716 and 726 ofFIG. 7) and encapsulation. In the example router 700, the example method300 consistent with the present disclosure may be implemented in thecontrol component 710, in the routing protocol processes 730.

The example control component 710 may provide several ways to manage therouter (e.g., for configuring the interval as discussed with referenceto block 305 of FIG. 3). For example, it 710 may provide a userinterface process(es) 760 which allows a system operator 765 to interactwith the system through configuration, modifications, and monitoring.The SNMP 785 allows SNMP-capable systems to communicate with the routerplatform. This also allows the platform to provide necessary SNMPinformation to external agents. For example, the SNMP 785 may permitmanagement of the system from a network management station runningsoftware, such as Hewlett-Packard's Network Node Manager (“HP-NNM”),through a framework, such as Hewlett-Packard's OpenView. Accounting ofpackets (generally referred to as traffic statistics) may be performedby the control component 710, thereby avoiding slowing trafficforwarding by the packet forwarding component 790.

Although not shown, the example router 700 may provide for out-of-bandmanagement, RS-232 DB9 ports for serial console and remote managementaccess, and tertiary storage using a removable PC card. Further,although not shown, a craft interface positioned on the front of thechassis provides an external view into the internal workings of therouter. It can be used as a troubleshooting tool, a monitoring tool, orboth. The craft interface may include LED indicators, alarm indicators,control component ports, and/or a display screen. Finally, the craftinterface may provide interaction with a command line interface (“CLI”)760 via a console port, an auxiliary port, and/or a management Ethernetport. As noted, the interval may be configured using the CLI.

The packet forwarding component 790 is responsible for properlyoutputting received packets as quickly as possible. If there is no entryin the forwarding table for a given destination or a given label and thepacket forwarding component 790 cannot perform forwarding by itself, it790 may send the packets bound for that unknown destination off to thecontrol component 710 for processing. The example packet forwardingcomponent 790 is designed to perform Layer 2 and Layer 3 switching,route lookups, and rapid packet forwarding.

As shown in FIG. 7, the example packet forwarding component 790 has anembedded microkernel 792, interface process(es) 793, distributed ASICs794, and chassis process(es) 795, and stores a forwarding (e.g.,route-based and/or label-based) table(s) 796. The microkernel 792interacts with the interface process(es) 793 and the chassis process(es)795 to monitor and control these functions. The interface process(es)792 has direct communication with the OS kernel 720 of the controlcomponent 710. This communication includes forwarding exception packetsand control packets to the control component 710, receiving packets tobe forwarded, receiving forwarding table updates, providing informationabout the health of the packet forwarding component 790 to the controlcomponent 710, and permitting configuration of the interfaces from theuser interface (e.g., CLI) process(es) 760 of the control component 710.The stored forwarding table(s) 796 is static until a new one is receivedfrom the control component 710. The interface process(es) 793 uses theforwarding table(s) 796 to look up next-hop information. The interfaceprocess(es) 793 also has direct communication with the distributed ASICs794. Finally, the chassis process(es) 795 may communicate directly withthe microkernel 792 and with the distributed ASICs 794.

Referring back to distributed ASICs 794 of FIG. 7, FIG. 8 is an exampleof how the ASICS may be distributed in the packet forwarding component790 to divide the responsibility of packet forwarding. As shown in FIG.8, the ASICs of the packet forwarding component 790 may be distributedon physical interface cards (“PICs”) 810, flexible PIC concentrators(“FPCs”) 820, a midplane or backplane 830, and a system control board(s)840 (for switching and/or forwarding). Switching fabric is also shown asa system switch board (“SSB”), or a switching and forwarding module(“SFM”) 850. Each of the PICs 810 includes one or more PIC I/O managers815. Each of the FPCs 820 includes one or more I/O managers 822, eachwith an associated memory 824. The midplane/backplane 830 includesbuffer managers 835 a, 835 b. Finally, the system control board 840includes an Internet processor 842 and an instance of the forwardingtable 844 (Recall, e.g., 796 of FIG. 7).

Still referring to FIG. 8, the PICs 810 contain the interface ports.Each PIC 810 may be plugged into an FPC 820. Each individual PIC 810 maycontain an ASIC that handles media-specific functions, such as framingor encapsulation. Some example PICs 810 provide SDH/SONET, ATM, GigabitEthernet, Fast Ethernet, and/or DS3/E3 interface ports.

An FPC 820 can contain from one or more PICs 810, and may carry thesignals from the PICs 810 to the midplane/backplane 830 as shown in FIG.8.

The midplane/backplane 830 holds the line cards. The line cards mayconnect into the midplane/backplane 830 when inserted into the examplerouter's chassis from the front. The control component (e.g., routingengine) 710 may plug into the rear of the midplane/backplane 830 fromthe rear of the chassis. The midplane/backplane 830 may carry electrical(or optical) signals and power to each line card and to the controlcomponent 710.

The system control board 840 may perform forwarding lookup. It 840 mayalso communicate errors to the routing engine. Further, it 840 may alsomonitor the condition of the router based on information it receivesfrom sensors. If an abnormal condition is detected, the system controlboard 840 may immediately notify the control component 710.

Referring to FIGS. 8, 9A and 9B, in some exemplary routers, each of thePICs 810,710′ contains at least one I/O manager ASIC 815 responsible formedia-specific tasks, such as encapsulation. The packets pass throughthese I/O ASICs on their way into and out of the router. The I/O managerASIC 815 on the PIC 810,710′ is responsible for managing the connectionto the I/O manager ASIC 822 on the FPC 820,720′, managing link-layerframing and creating the bit stream, performing cyclical redundancychecks (CRCs), and detecting link-layer errors and generating alarms,when appropriate. The FPC 820 includes another I/O manager ASIC 822.This ASIC 822 takes the packets from the PICs 810 and breaks them into(e.g., 64-byte) memory blocks. This FPC I/O manager ASIC 822 sends theblocks to a first distributed buffer manager (DBM) 835 a′, decodingencapsulation and protocol-specific information, counting packets andbytes for each logical circuit, verifying packet integrity, and applyingclass of service (CoS) rules to packets. At this point, the packet isfirst written to memory. More specifically, the example DBM ASIC 835 a′manages and writes packets to the shared memory 824 across all FPCs 820.In parallel, the first DBM ASIC 835 a′ also extracts information on thedestination of the packet and passes this forwarding-related informationto the Internet processor 842/742′. The Internet processor 842/742′performs the route lookup using the forwarding table 844 and sends theinformation over to a second DBM ASIC 835 b′. The Internet processorASIC 842/742′ also collects exception packets (i.e., those without aforwarding table entry) and sends them to the control component 710. Thesecond DBM ASIC 835 b′ then takes this information and the 64-byteblocks and forwards them to the I/O manager ASIC 822 of the egress FPC820/720′ (or multiple egress FPCs, in the case of multicast) forreassembly. (Thus, the DBM ASICs 835 a′ and 835 b′ are responsible formanaging the packet memory 824 distributed across all FPCs 820/720′,extracting forwarding-related information from packets, and instructingthe FPC where to forward packets.)

The I/O manager ASIC 822 on the egress FPC 820/720′ may perform somevalue-added services. In addition to incrementing time to live (“TTL”)values and re-encapsulating the packet for handling by the PIC 810, itcan also apply class-of-service (CoS) rules. To do this, it may queue apointer to the packet in one of the available queues, each having ashare of link bandwidth, before applying the rules to the packet.Queuing can be based on various rules. Thus, the I/O manager ASIC 822 onthe egress FPC 820/720′ may be responsible for receiving the blocks fromthe second DBM ASIC 835 b′, incrementing TTL values, queuing a pointerto the packet, if necessary, before applying CoS rules, re-encapsulatingthe blocks, and sending the encapsulated packets to the PIC I/O managerASIC 815.

FIG. 10 is a flow diagram of an example method 1000 for providing packetforwarding in the example router. The main acts of the method 1000 aretriggered when a packet is received on an ingress (incoming) port orinterface. (Event 1010) The types of checksum and frame checks that arerequired by the type of medium it serves are performed and the packet isoutput, as a serial bit stream. (Block 1020) The packet is thendecapsulated and parsed into (e.g., 64-byte) blocks. (Block 1030) Thepackets are written to buffer memory and the forwarding information ispassed on the Internet processor. (Block 1040) The passed forwardinginformation is then used to lookup a route in the forwarding table.(Block 1050) (Recall, e.g., FIGS. 8A-8D.) Note that the forwarding tablecan typically handle unicast packets that do not have options (e.g.,accounting) set, and multicast packets for which it already has a cachedentry. Thus, if it is determined that these conditions are met (YESbranch of Decision 1060), the packet forwarding component finds the nexthop and egress interface, and the packet is forwarded (or queued forforwarding) to the next hop via the egress interface (Block 1070) beforethe method 1000 is left (Node 1090) Otherwise, if these conditions arenot met (NO branch of Decision 1060), the forwarding information is sentto the control component 710 for advanced forwarding resolution (Block1080) before the method 1000 is left (Node 1090).

Referring back to block 1070, the packet may be queued. Actually, asstated earlier with reference to FIG. 8, a pointer to the packet may bequeued. The packet itself may remain in the shared memory. Thus, allqueuing decisions and CoS rules may be applied in the absence of theactual packet. When the pointer for the packet reaches the front of theline, the I/O manager ASIC 822 may send a request for the packet to thesecond DBM ASIC 835 b. The DBM ASIC 835 reads the blocks from sharedmemory and sends them to the I/O manager ASIC 822 on the FPC 820, whichthen serializes the bits and sends them to the media-specific ASIC ofthe egress interface. The I/O manager ASIC 815 on the egress PIC 810 mayapply the physical-layer framing, perform the CRC, and send the bitstream out over the link.

Referring back to block 1080 of FIG. 10, as well as FIG. 8, regardingthe transfer of control and exception packets, the system control board840 handles nearly all exception packets. For example, the systemcontrol board 840 may pass exception packets to the control component710.

Although example embodiments consistent with the present disclosure maybe implemented on the example routers of FIG. 6 or 7, embodimentsconsistent with the present disclosure may be implemented oncommunications network nodes (e.g., routers, switches, etc.) havingdifferent architectures. For example, methods consistent with thepresent description may be run on any modern server, or even a virtualmachine (e.g., without any ASICs or packet forwarding engines). Moregenerally, embodiments consistent with the present disclosure may beimplemented on an example system 1100 as illustrated on FIG. 11.

FIG. 11 is a block diagram of an exemplary machine 1100 that may performone or more of the methods described, and/or store information usedand/or generated by such methods. The exemplary machine 1100 includesone or more processors 1110, one or more input/output interface units1130, one or more storage devices 1120, and one or more system busesand/or networks 1140 for facilitating the communication of informationamong the coupled elements. One or more input devices 1132 and one ormore output devices 1134 may be coupled with the one or moreinput/output interfaces 1130. The one or more processors 1110 mayexecute machine-executable instructions (e.g., C or C++ running on theLinux operating system widely available from a number of vendors) toeffect one or more aspects of the present disclosure. At least a portionof the machine executable instructions may be stored (temporarily ormore permanently) on the one or more storage devices 1120 and/or may bereceived from an external source via one or more input interface units1130. The machine executable instructions may be stored as varioussoftware modules, each module performing one or more operations.Functional software modules are examples of components, which may beused in the apparatus described.

In some embodiments consistent with the present disclosure, theprocessors 1110 may be one or more microprocessors and/or ASICs. The bus1140 may include a system bus. The storage devices 1120 may includesystem memory, such as read only memory (ROM) and/or random accessmemory (RAM). The storage devices 1120 may also include a hard diskdrive for reading from and writing to a hard disk, a magnetic disk drivefor reading from or writing to a (e.g., removable) magnetic disk, anoptical disk drive for reading from or writing to a removable (magneto-)optical disk such as a compact disk or other (magneto-) optical media,or solid-state non-volatile storage.

Some example embodiments consistent with the present disclosure may alsobe provided as a machine-readable medium for storing themachine-executable instructions. The machine-readable medium may benon-transitory and may include, but is not limited to, flash memory,optical disks, CD-ROMs, DVD ROMs, RAMs, EPROMs, EEPROMs, magnetic oroptical cards or any other type of machine-readable media suitable forstoring electronic instructions. For example, example embodimentsconsistent with the present disclosure may be downloaded as a computerprogram, which may be transferred from a remote computer (e.g., aserver) to a requesting computer (e.g., a client) by way of acommunication link (e.g., a modem or network connection) and stored on anon-transitory storage medium. The machine-readable medium may also bereferred to as a processor-readable medium.

Example embodiments consistent with the present disclosure (orcomponents or modules thereof) might be implemented in hardware, such asone or more field programmable gate arrays (“FPGA”s), one or moreintegrated circuits such as ASICs, one or more network processors, etc.Alternatively, or in addition, embodiments consistent with the presentdisclosure (or components or modules thereof) might be implemented asstored program instructions executed by a processor. Such hardwareand/or software might be provided in an addressed data (e.g., packet,cell, etc.) forwarding device (e.g., a switch, a router, etc.), a laptopcomputer, desktop computer, a tablet computer, a mobile phone, or anydevice that has computing and networking capabilities.

§ 4.4 Refinements, Alternatives and Extensions

Although the example embodiments were described in the context of a datacenter environment in which a host device is multihomed via two leafnodes, the use of such example embodiments is not limited to such anenvironment. That is, example embodiments consistent with the presentdescription may be used in other multihomed architectures, in which afault detection protocol (e.g., BFD) is run over a LAG.

Although the multicast destination IP address 224.0.0.120 was used topermit the node to understand that the message is for a multihomed LAGbundle, another otherwise unassigned multicast IP address (e.g.,224.0.0.X) may be used instead.

Although the multicast MAC address (e.g., 01-00-5E-90-00-01) was used inorder to be compliant with RFC 7130, if another MAC destination addressis needed for compliance with some other fault detection protocol orRFC, such a different MAC destination address may be used instead.

Referring back to 550 of FIG. 5, although the destination port was setto 6784 for compliance with RFC 7130, if a different destination port isneeded for compliance with some other fault detection protocol or RFC,such a different destination port may be used instead.

Similarly, referring back to 590 of FIG. 5, another fault detectionprotocol packet can be used instead of a BFD control packet or a BFDecho packet.

§ 4.5 Conclusions

Example embodiments consistent with the present description support afault detection protocol (e.g., BFD) in a multihomed environment, suchas a multihomed LAG for example, by ensuring that the destinationaddress, which is used to send a fault detection protocol packet, isaccepted even without being configured on the peering nodes. Since thefault detection protocol packet would not be directed to particulardestination address, it is important to use an appropriate destinationMAC address. For example, in the context of BFD, the same dedicated MACaddress (i.e., 01-00-5E-90-00-01) as defined in RFC 7130 may be used.Using a multicast reserved destination MAC address would be in line witha destination MAC where the multicast bit is set. Using a reservedmulticast IP destination address and a reserved multicast MACdestination address allows a multihomed host node (or some othermultihomed node, such as a customer edge (CE) node for example) to sendfault detection protocol packets to different connected leaf nodes (orsome other nodes, such as provider edge (PE) nodes for example).

Upon receiving such a fault detection packet, a node (e.g., a leaf node)should process the packet normally, without checking the source IPaddress. For example, in the context of BFD, this is because RFC 5581explicitly mentions that on a point-to-point interface, the source IPaddress of incoming BFD packet should not be tied with any BFD session.That is, in most implementations, BFD packets could be checked forsource address and compared with the configured peer address. If theaddresses don't match, then the packet(s) would be discarded.

The example embodiments consistent with the present description allowfault detection protocol (e.g., BFD) session establishment on multihomedenvironment with directly connected interfaces, such as between a hostnode and data center leaf nodes.

What is claimed is:
 1. For use in a system including a first dataforwarding device, a second data forwarding device, a third dataforwarding device, a first communications link between the first dataforwarding device and the second data forwarding device, and a secondcommunications link between the first data forwarding device and thethird data forwarding device, the first and second communications linksbelonging to a link aggregation group (LAG), a computer implementedmethod comprising: a) generating a message i) for testing (1) a firstpath between the first data forwarding device and the second dataforwarding device, and (2) a second path between the first dataforwarding device and the third data forwarding device, and ii)including a payload containing path testing information; b) sending,over the LAG, the message from the first data forwarding device to boththe second data forwarding device and the third data forwarding device;c) receiving, by the second data forwarding device, an instance of themessage; d) responsive to receiving the instance of the message by thesecond data forwarding device, either (A) (1) checking, with the seconddata forwarding device, the instance of the message for compliance witha path testing procedure, and (2) responsive to determining that theinstance of the message is compliant with the path testing procedure,processing, with the second data forwarding device, the instance of themessage in accordance with the path testing procedure and otherwise,responsive to determining that the instance of the message is notcompliant with the path testing procedure, dropping by the second dataforwarding device, the instance of the message, or (B) (1) determining,by the second data forwarding device, whether or not the instance of themessage is a fault detection on a multihomed link aggregation groupmessage, and (2) processing, by the second data forwarding device, theinstance of the message based on the determination of whether or not itis the fault detection on a multihomed link aggregation group message;e) receiving, by the third data forwarding device, another instance ofthe message; and f) responsive to receiving the other instance of themessage by the third data forwarding device, either (A) (1) checking,with the third data forwarding device, the other instance of the messagefor compliance with a path testing procedure, and (2) responsive todetermining that the other instance of the message is compliant with thepath testing procedure, processing, with the third data forwardingdevice, the other instance of the message in accordance with the pathtesting procedure and otherwise, responsive to determining that theother instance of the message is not compliant with the path testingprocedure, dropping by the third data forwarding device, the otherinstance of the message, or (B) (1) determining, by the third dataforwarding device, whether or not the other instance of the message is afault detection on a multihomed link aggregation group message, and (2)processing, by the third data forwarding device, the other instance ofthe message based on the determination of whether or not it is the faultdetection on a multihomed link aggregation group message.
 2. Thecomputer-implemented method of claim 1 wherein the message includes amulticast Internet Protocol (IP) destination address and wherein themessage is carried in an Ethernet frame.
 3. The computer-implementedmethod of claim 2 wherein the Ethernet frame includes a multicastdestination media access control (MAC) address.
 4. Thecomputer-implemented method of claim 3 wherein the multicast destinationMAC address is 01-00-5E-90-00-01.
 5. The computer-implemented method ofclaim 1 wherein the path testing information in the payload of themessage includes at least one of (A) a bidirectional forwardingdetection (BFD) control packet compliant with the “BidirectionalForwarding Detection (BFD),” Request for Comments (RFC) 5880 issued June2010 or (B) a BFD echo packet compliant with RFC
 5880. 6. Thecomputer-implemented method of claim 5 wherein the path testinginformation in the payload of the message includes at least one of (A) abidirectional forwarding detection (BFD) control packet compliant withthe “Bidirectional Forwarding Detection (BFD) on Link Aggregation Group(LAG) Interfaces,” Request for Comments (RFC) 7130, from the InternetEngineering Task Force, issued February 2014 or (B) a BFD echo packetcompliant with RFC
 7130. 7. The computer-implemented method of claim 1,wherein responsive to determining that the message is compliant with thepath testing procedure, processing by the second and third dataforwarding devices, the message in accordance with the path testingprocedure and otherwise, responsive to determining that the message isnot compliant with the path testing procedure, dropping by the secondand third data forwarding devices, the message.
 8. Thecomputer-implemented method of claim 7 wherein the act of testing themessage for compliance with the path testing procedure includes checkinga destination IP address of the message.
 9. The computer-implementedmethod of claim 7 wherein the path testing information in the payload ofthe message includes at least one of (A) a bidirectional forwardingdetection (BFD) control packet compliant with the “BidirectionalForwarding Detection (BFD),” Request for Comments (RFC) 5880 from theInternet Engineering Task Force issued June 2010 or (B) a BFD echopacket compliant with RFC 5880, and wherein the path testing procedureis bidirectional forwarding detection in accordance with RFC
 5880. 10.The computer-implemented method of claim 7 wherein the path testinginformation in the payload of the message includes at least one of (A) abidirectional forwarding detection (BFD) control packet compliant withthe “Bidirectional Forwarding Detection (BFD),” Request for Comments(RFC) 5880 from the Internet Engineering Task Force issued June 2010 or(B) a BFD echo packet compliant with RFC 5880, and wherein the pathtesting procedure is bidirectional forwarding detection in accordancewith the “Bidirectional Forwarding Detection (BFD) on Link AggregationGroup (LAG) Interfaces,” Request for Comments (RFC) 7130, from theInternet Engineering Task Force, issued February
 2014. 11. Thecomputer-implemented method of claim 1 wherein, responsive to receivingthe instance of the message by the second data forwarding device, 1)determining, by the second data forwarding device, whether or not theinstance of the message is a fault detection on a multihomed linkaggregation group message, and 2) processing, by the second dataforwarding device, the instance of the message based on thedetermination of whether or not it is the fault detection on amultihomed link aggregation group message, and responsive to receivingthe other instance of the message by the third data forwardingdevice, 1) determining, by the third data forwarding device, whether ornot the other instance of the message is a fault detection on amultihomed link aggregation group message, and 2) processing, by thethird data forwarding device, the other instance of the message based onthe determination of whether or not it is the fault detection on amultihomed link aggregation group message.
 12. A system comprising: a) afirst data forwarding device; b) a second data forwarding device; c) athird data forwarding device; d) a first communications link between thefirst data forwarding device and the second data forwarding device; ande) a second communications link between the first data forwarding deviceand the third data forwarding device, wherein the first and secondcommunications links belonging to a link aggregation group (LAG),wherein the first data forwarding device is configured to generate amessage i) for testing (1) a first path between the first dataforwarding device and the second data forwarding device, and (2) asecond path between the first data forwarding device and the third dataforwarding device, and ii) including a payload containing path testinginformation, send, over the LAG, an instance of the message to thesecond data forwarding device, and send, over the LAG, another instanceof the message to the third data forwarding device, wherein the seconddata forwarding device is configured to receive the instance of themessage, and responsive to receiving the instance of the message, either(A) (1) check the instance of the message for compliance with a pathtesting procedure, and (2) responsive to determining that the instanceof the message is compliant with the path testing procedure, process theinstance of the message in accordance with the path testing procedureand otherwise, responsive to determining that the instance of themessage is not compliant with the path testing procedure, drop theinstance of the message, or (B) (1) determine whether or not theinstance of the message is a fault detection on a multihomed linkaggregation group message, and (2) process the instance of the messagebased on the determination of whether or not it is the fault detectionon a multihomed link aggregation group message, and wherein the thirddata forwarding device is adapted to receive the other instance of themessage, and responsive to receiving the other instance of the message,either (A) (1) check the other instance of the message for compliancewith a path testing procedure, and (2) responsive to determining thatthe other instance of the message is compliant with the path testingprocedure, process the other instance of the message in accordance withthe path testing procedure and otherwise, responsive to determining thatthe other instance of the message is not compliant with the path testingprocedure, drop the other instance of the message, or (B) (1) determinewhether or not the other instance of the message is a fault detection ona multihomed link aggregation group message, and (2) process the otherinstance of the message based on the determination of whether or not itis the fault detection on a multihomed link aggregation group message.13. The system of claim 12 wherein the message includes a multicastInternet Protocol (IP) destination address and wherein the message iscarried in an Ethernet frame.
 14. The system of claim 13 wherein theEthernet frame includes a multicast destination media access control(MAC) address.
 15. The system node of claim 14 wherein the multicastdestination MAC address is 01-00-5E-90-00-01.
 16. The system of claim 12wherein the path testing information in the payload of the bidirectionalforwarding on multihomed LAG message includes at least one of (A) abidirectional forwarding detection (BFD) control packet compliant withthe “Bidirectional Forwarding Detection (BFD),” Request for Comments(RFC) 5880 from the Internet Engineering Task Force issued June 2010 or(B) a BFD echo packet compliant with RFC
 5880. 17. The system of claim12 wherein the path testing information in the payload of thebidirectional forwarding on multihomed LAG message includes at least oneof (A) a bidirectional forwarding detection (BFD) control packetcompliant with the “Bidirectional Forwarding Detection (BFD) on LinkAggregation Group (LAG) Interfaces,” Request for Comments (RFC) 7130,from the Internet Engineering Task Force, issued February 2014 or (B) aBFD echo packet compliant with RFC
 7130. 18. A non-transitorycomputer-readable medium storing processor executable instructionswhich, when executed by a least one processor, perform a method for usein a system including a first data forwarding device, a second dataforwarding device, a third data forwarding device, a firstcommunications link between the first data forwarding device and thesecond data forwarding device, and a second communications link betweenthe first data forwarding device and the third data forwarding device,the first and second communications links belonging to a linkaggregation group (LAG), the method comprising: a) generating a messagei) for testing (1) a first path between the first data forwarding deviceand the second data forwarding device, and (2) a second path between thefirst data forwarding device and the third data forwarding device, andii) including a payload containing path testing information; and b)sending, over the LAG, the message from the first data forwarding deviceto both the second data forwarding device and the third data forwardingdevice; c) receiving, by the second data forwarding device, an instanceof the message; d) responsive to receiving the instance of the messageby the second data forwarding device, either (A) (1) checking, with thesecond data forwarding device, the instance of the message forcompliance with a path testing procedure, and (2) responsive todetermining that the instance of the message is compliant with the pathtesting procedure, processing, with the second data forwarding device,the instance of the message in accordance with the path testingprocedure and otherwise, responsive to determining that the instance ofthe message is not compliant with the path testing procedure, droppingby the second data forwarding device, the instance of the message, or(B) (1) determining, by the second data forwarding device, whether ornot the instance of the message is a fault detection on a multihomedlink aggregation group message, and (2) processing, by the second dataforwarding device, the instance of the message based on thedetermination of whether or not it is the fault detection on amultihomed link aggregation group message; e) receiving, by the thirddata forwarding device, another instance of the message; and f)responsive to receiving the other instance of the message by the thirddata forwarding device, either (A) (1) checking, with the third dataforwarding device, the other instance of the message for compliance witha path testing procedure, and (2) responsive to determining that theother instance of the message is compliant with the path testingprocedure, processing, with the third data forwarding device, the otherinstance of the message in accordance with the path testing procedureand otherwise, responsive to determining that the other instance of themessage is not compliant with the path testing procedure, dropping bythe third data forwarding device, the other instance of the message, or(B) (1) determining, by the third data forwarding device, whether or notthe other instance of the message is a fault detection on a multihomedlink aggregation group message, and (2) processing, by the third dataforwarding device, the other instance of the message based on thedetermination of whether or not it is the fault detection on amultihomed link aggregation group message.
 19. The non-transitorycomputer-readable medium of claim 18, wherein responsive to determiningthat the message is compliant with the path testing procedure,processing by the second and third data forwarding devices, the messagein accordance with the path testing procedure and otherwise, responsiveto determining that the message is not compliant with the path testingprocedure, dropping by the second and third data forwarding devices, themessage.
 20. The non-transitory computer-readable medium of claim 18,wherein responsive to receiving the instance of the message by thesecond data forwarding device, 1) determining whether or not theinstance of the message is a fault detection on a multihomed linkaggregation group message, and 2) processing the instance of the messagebased on the determination of whether or not it is the fault detectionon a multihomed link aggregation group message, and responsive toreceiving the other instance of the message by the third data forwardingdevice, 1) determining whether or not the other instance of the messageis a fault detection on a multihomed link aggregation group message, and2) processing the other instance of the message based on thedetermination of whether or not it is the fault detection on amultihomed link aggregation group message.